1. Field of Invention
The present invention relates to broadband wireless access systems, and amongst other things to an adaptive downstream modulation scheme for use in broadband wireless access systems.
2. Discussion of Background
Point to multipoint fixed broadband wireless access systems over MMDS networks are known in broadcast situations. These networks operate over licensed bands including the MMDS band (2,150 to 2,162 MHz), the WCS band (2,305 to 2,311 MHz) and the ITFS/MMDS bands (2,500 to 2,686 MHz).
A known wireless broadband access system, which operates at a range of between 50 MHz and 864 MHz, but not in the MMDS, WCS, or ITFS/MMDS bands, is the data over cable specification system, which is specified in the data over cable system interface specifications (DOCSIS). An overview of a wireless DOCSIS system is depicted in FIG. 1. A CMTS 10 communicates with a wide area network 20, such as the Internet. The CMTS 10 can transmit signals from the wide area network 20 along a cable network 30 through cable modems 40 to CPE 50. CPE 50 messages can be transmitted to the wide area network 20 through the cable modem 40 along the cable network 30 to the CMTS 10.
In point to multipoint broadband wireless access systems one central end-point, e.g. the head-end, communicates through a bi-directional link or links with multiple end-points, e.g. the nodes. The number of nodes in communication varies in time and can be none, one or two or more at any specific time.
The link(s) between the head-end and the nodes are combined in one or more channels. The signal path from the central end-point to the nodes is referred to as downstream, while the signal path from the nodes to the central end-point is referred to as upstream.
In such a system the diverse topological conditions of each user within the service area in terms of factors such as signal to noise, multipath, and fading make it difficult to implement a flexible communication scheme that will enable per user optimization with respect to robustness and throughput. Further, these conditions vary over time, which means that the characteristics of downstream channels change over time, which further adds to the difficulty of maintaining quality and flexible communication over wireless networks.
As the characteristics of the downstream channel change, wireless communication systems have to attempt to balance two competing factors to improve communication. The major concern for all data systems operators is the improvement and increase of the data throughput of the system. In single carrier systems, the data throughput is generally increased by using a higher order modulation, e.g. QAM 64 has a higher data capacity than QAM 16. However, if there are problems in the transmission path or with a wireless receiver, the operator of the system should attempt to increase the robustness of the communication path in order to prevent packet loss and decrease error rates. Robustness is increased by reducing the data communication rate, which is generally performed by decreasing the modulation type, e.g. moving from QAM 16 to QPSK. As can be seen from the above discussion, increasing the modulation type results in a lower robustness and a decrease in the modulation type causes a decrease in data throughput making the selection of the modulation type a very important factor in the implementation of wireless communication systems.
Therefore, there exists a need for an effective way to both maximizes the data throughput and the robustness of wireless communication systems.